Natural fibers, including cellulose containing natural fibers from plants, such as cotton and linen, have been used by mankind for more than 5000 years. Natural cellulose containing fibers, however, do not possess the chemical versatility of synthetic fibers, due to the relative inert nature of the cellulose consisting of β-1-4 linked glucose monomers.
This relatively inert nature is e.g. apparent during the dyeing process of cotton fibers and fabrics. Several types of dyes are used to color cotton, such as direct dyes and, most importantly, fiber-reactive dyes, which are both anionic molecules. Cotton itself develops an anionic charge in water, so that without special treatment, the uptake of dye by the fiber or fabric is quite elaborate.
Direct dyes create a relatively weak hydrogen bond with the cellulose polymer forming a semi-permanent attachment. Direct dyes are easier to use and less expensive than fiber-reactive dyes, but do not withstand well washing. Fiber-reactive dyes are molecules that combine chromophores with a reactive group that forms strong covalent bonds with the fiber via reaction with hydroxyl groups. The covalent bonds provide a good resistance of the dyed fiber against laundering.
During the dyeing process, large amounts of electrolytes are needed to shield the anionic dyes from the anionic fiber charges. Unreacted hydrolyzed dyes (up to 40%) need to be removed by multiple washing steps, generating large volumes of wastewater, also containing the above mentioned electrolytes.
Providing the cellulose fiber with a positive electric charge, e.g. by incorporation of positively charged chemical compounds, could therefore improve the dyeability of natural cellulose fibers, as well as improve any chemical reaction of the modified cellulose fiber with negatively charged chemical compounds. It would also make the use of acidic dyes possible.
Several publications have described the incorporation into or coating of chitosan oligomers into cellulose fibers to make chitosan/cellulose blends, yarns or fabrics. Chitosan is a positively charged polymer of glucosamine, which can be obtained by deacetylation of chitin, e.g. by alkalic treatments. Chitin itself is a polymer of β-1-4 linked N-acetylglucosamine (GlcNAc).
US patent application US2003/0134120 describes the coating of natural fibers with chitosan.
Liu et al. (Carbohydrate Polymers 44(2003) 233-238) describe a method for coating cotton fibers with chitosan, by oxidation of the cotton thread with potassium periodate at 60° C. in water and subsequent treatment with a solution of chitosan in aqueous acetic acid. With the chitosan coating, the cotton fiber surface became physiologically and biologically active. Since the chemical reactivity of the amino group is greater than the hydroxyl group of cellulose monomers, the fiber has more potential for further chemical modification. Moreover, the smooth surface of the cotton fiber became coarse, suggesting a greater potential for drug absorption and controlled release thereof.
Based on the physiological function of chitosan in inhibiting e.g. dermatophytes, many functional clothes, fabrics and fibers employ cellulose-chitosan blend fibers, cellulose fiber-chitosan conjugates and fabrics coated with chitosan-containing resins.
WO 00/09729 describes the expression of chitin synthase and chitin deacetylase genes in plants to alter the cell wall for industrial uses and improved disease resistance. Specifically cited uses are: to provide a single plant source of cellulose, chitin and chitosan, to increase tensile strength and to increase brittle snap. Specifically suggested chitin synthase genes are derived from fungal organisms. No experimental data are provided on the production of chitin or chitosan in plants, nor on the incorporation thereof in plant cell walls.
WO2006/136351 showed that the strategy as proposed in WO00/09729 does not lead to the functional incorporation of chitin into the plant cell wall. Instead, WO 2006/136351 discloses that chitin is effectively produced in the secondary cell wall of cotton fibers only when the N-acetylglucosamine transferase is relocated to the Golgi apparatus. For the fungal chitin synthase from Neurospora crassa, relocation to the Golgi apparatus is achieved by operable fusion of this fungal chitin synthase with a heterologous signal anchor sequence specific for the Golgi apparatus, and by expressing the resulting chimeric gene in plants. For the NODC type of N-acetylglucosamine transferase however, addition of a signal anchor sequence is not required for localization of the NodC protein to the Golgi apparatus, and for incoporation of chito-oligosaccharides into the plant cell wall without external GlcNAc feeding. Although chitin could be efficiently produced in the plant cell walls, it was also observed that transgenic plants comprising NODC had shorter roots as compared to wild-type plants.
Thus there remains a need for alternative methods to produce plant cell walls such as secondary cell walls which comprise positively charged polysaccharides. In particular a need exists for providing methods to produce plants with positively charged oligosaccharides in their cell walls, but without root growth retardation. These and other problems are solved as described hereinafter in the different embodiments, examples and claims.